AN APPARATUS FOR AND A METHOD OF FILTERING GASES

Abstract: Gas with entrained particles, such as high temperature and pressure gas from a pressurized circulating fluidized bed reactor, is filtered using a vessel having a gas inlet (4), cleaned gas outlet (5), and filtered particles outlet (13). A pulse of cleaning gas is periodically or intermittently directed to the filters (9) to dislodge the particles which have been filtered and collected on the filters (9). Shrouds (10), having tubular, parallelepiped, zig-zag, or honeycomb configurations, are provided surrounding relevant portions of the filters to prevent dislodged particles from being entrained in gas passing to other filters, and instead directing the dislodged particles toward the vessel particles outlet. The shroud walls may have openings in them to allow gas to be filtered to pass through them, but prevent dislodged particles from passing through the openings. The filters (9) may be mounted in hollow tubular or parallelepiped ducts (12) in such a way that the ducts have smooth outer walls, and the shrouds (10) may be substantially concentric with the ducts. When individual cleaning gas pulsing devices are associated with filters, the shrouds may include separate impingement and material collecting portions.

Description: AN APPARATUS FOR AND A METHOD OF FILTERING GASES BACKGROUND OF THE INVENTION The present invention relates to an apparatus for filte¬ ring gases and a method of operating an apparatus for filte- ring gases. More specifically, the invention relates to an apparatus and a method for filtering gases containing solid contaminants and further to removing accumulated solids from filtering elements in a smooth and safe manner. In various modern applications the efficiency of filte- ring devices is required to be extremely high, while the pressure drop over the filtering device must not exceed a certain predetermined level, and thus more and more effective cleaning systems are established. High temperature durability systems are also required for some applications. A Pressuri- zed Fluidized Bed combustor connected to a gas compres¬ sor/turbine system can be considered such. In order to be able to use solid fuel, such as coal or peat, for generating pressurized hot flue gases to be expanded in a gas turbine, one must have appropriate means for preventing the particu- late matter carry-over from combustion of such solid fuels. An efficient system for filtering the gases would permit the use of such fuels. Removal of particulate matter from combustor exhaust gas effluent stream before leading the gas stream to a turbine can prolong the life of turbine blades and thus increase the reliability and efficiency of the whole process involved. Also, in some other cases, the governmental regulations regarding emissions permitted due to public health may cont¬ ribute to the efforts to provide a system for filtering contaminated gases more efficiently. In prior art, it is known to use various kinds of fil¬ ters, such as bag house filters or ceramic filters for hot environment, in order to remove particulate from the gas stream. It is also essential to have the filter cleaned after a certain pre-determined operating period in order to main- tain cleaning efficiency and pressure reduction of the gas over the filter in desired range. A commonly used method of cleaning the filter is using a reverse pulse of gas for flushing the filter. This kind of method is disclosed in, e.g., U.S patent 4,764,190. The filter elements are cleaned by use of a back-pulse manifold that directs a pulse of gas flow into the filter elements in a direction opposite to normal direction of gas flow to be filtered, thus dislodging the accumulated cake previously separated from the gas. Also, European patent 0057 251 discloses a filter having a plurality of filter elements supported by a tube sheet, each filter element having open tops disposed in apertures of the tube sheet extending into lower chamber of the filter. There is also disclosed a system for backflushing the filter elements. The filter elements are cleaned by a pulse of high pressure gas from a gas delivering pipe, with help of a venturi the gas pulse pulls additional gas from the immediate vicinity and increases the total amount of the cleaning gas improving the cleaning effect. Further, it is stated that the system can include features which alleviate any undesirable interaction between the dirty gas entering the vessel and the downwardly flowing purged particles. According to the refe¬ rence, the inlets for the gas to be cleaned are positioned high in the vessel,, to promote a generally downwardly flow. EP 0 057 251 also discloses a baffle having perforations in upper section, the baffle being arranged to form an annular space between a wall of the vessel, and functioning as an equalizer of the inlet flow of the gas to be distributed evenly to the filter elements, thus the gas flow is arranged to be generally downwardly near the inner surface of the baffle. The baffle extends from the bottom of the vessel to the upper section thereof. EP-B1-0078 678 discusses, in more detail, the dislodging of particulates from filtering fabric by assistance of indu- ced gas flow on the upstream side ( ie. outer side ) on the filter element. Gas is forced to circulate upwards along the outer surface of the filter fabric, thus maintaining the amount of particles in the gas stream. Although the method may be feasible to some extent but basically the suggested inducement of the gas surrounding the filter element causes more troubles by not allowing the dislodged particles to fall down. The back-pulse flushing is commonly practised so that only one or a few filters are cleaned at same time while the others are in normal usage mode in order to ensure a proper running of the process while maintaining the pressure reduc- tion within permitted range. When activating the back pulse system for cleaning a filter during a cleaning mode, the gas pulse is introduced into the filter causing the cake of particulate material accumulated on the surface of the filter to dislodge and be entrained in the surrounding gas space by the force of the back pulse. However, since there are filters in normal usage, i.e. sucking gas from the plenum into and through the filter, in vicinity of the pulsed filter, the dislodged particulate is strongly entrained by those gases and thus easily carried into another filter. Thus the efficiency of the whole filter¬ ing device is deteriorated as a result of the above mentioned phenomenon. The problem causes a number of difficulties, e.g. the sequence of flushing the filters must be multiplied, causing the decreasing of efficient filtering area available for filtering, the density of particulate in the gas is generally increased in filtering plenum, causing unnecessary abrasion of surfaces, which can also lead to problems eg. if cooled constructions are used in the filter. SUMMARY OF THE INVENTION It is an object of the present invention to provide a filtering apparatus capable of effecting filtering gases. It is a further object of the present invention to provide a method of and an apparatus for filtering gases in high tempe¬ rature and high pressure circumstances. The present invention solves the above discussed problems, related to commonly used practise of flushing the filter elements used for cleaning contaminated gases, with a novel and surprisingly efficient manner. A characteristic feature of an apparatus according to the present invention is that the apparatus comprises a vessel having a first plenum for gases containing contami¬ nants to be removed from the gases and a second plenum for receiving gas from filtering means, at least one inlet for introducing gases to the plenum for gases containing con¬ taminants, at least one outlet for filtered gases, at least one outlet for solids separated from gases, at least one filtering means adapted in the vessel forming a flow path for gases from the first plenum to the second plenum for recei¬ ving gas from the filtering means, the filtering means having at least one element substantially permeable to gases and substantially impermeable to solids, through which element the gases are arranged to flow, means for supplying a reverse gas pulse into the filtering means for removing the accumula¬ ted solids separated from the gases flowing through the filtering means, and a shroud means covering at least part of the filtering means for preventing the re-entrainment of the solids removed from the filtering means and enhancing the migration of solids to a region where the solids are not influenced by flow of the gases. The characteristic features of the method according to the present invention for filtering gases utilizing an appa¬ ratus of the present invention are introducing the gases con¬ taining contaminants to be removed via an inlet into a first plenum of said vessel, arranging said gases to flow through said filtering means, thus performing a filtering of said gases, separating and accumulating at least a portion of the contaminants from the gases into said filtering means, di¬ scharging the filtered gases via the outlet for further processing, arranging a flushing reverse gas pulse into at least one filtering means for removing the accumulated solids separated from the gases, arranging at least a portion of the removed solids to impact a shroud means isolating the cloud of backflushed solids, and arranging said solids to be con- veyed by said shroud means towards lower portion of said vessel. According to the present invention it is possible to provide a filtering device for sustained running periods combined with efficient cleaning facilities by utilizing the method of the present invention. Since the basic construction of the filter may be designed according to the needs of the process involved, eg. when connected to a Pressurized Flui- dized Bed Combustor/Gasifier the filter may be constructed of pressure/temperature proof materials and formed into shape of most optimum performance, the more efficient filtering pro¬ cess may be gained according to the present invention without loss of performance. And, since the preferred embodiment of the present invention utilizes plenums in the vessel for treating the gas, the maintenance and utilization of filte¬ ring means is most conveniently performed. According to one preferred embodiment of the invention the filtering system is resistant for high temperature and high pressure circumstances, being operatively connectable to a Pressurized Fluidized Bed reactor, such as Pressurized Fluidized Bed combustor of gasifier, for receiving and pro¬ cessing the gases resulting from the reactions taking place in such a reactor. Due to eg. the reliable performance of long running applications the filtering system of the present invention is most applicable to above mentioned systems or the like. The gases originating from such a reactor are usually in a pressure of 2 to 40 bar and at a temperature of 500 to 1200°C in which circumstances the filtering system of the present invention results in problem-free running facili- ties. According to a preferred embodiment of the invention, the filtering system is preferably formed of a plurality of clusters of filter elements, each cluster formed of a plura¬ lity of filter elements, preferably of ceramic material. The filtering system comprises an assembly for supplying a rever- se-flushing gas pulse into each cluster of filter elements, preferably separately, in order to clean the filter elements by dislodging the accumulated solids. The cluster of filter elements is provided with a shroud means surrounding the cluster, said cluster being preferably arranged in a vessel extending substantially vertically from a substantially horizontal supporting sheet. The shroud means for preventing the re-entrainment of the solids removed from the filter ele¬ ments is preferably a substantially vertical tubular member arranged to enclose the cluster of filters for collecting the solids flushed from the filter during the back-pulse opera¬ tion and for conveying the solids towards the lower section. In order to maintain the gas velocity within the permitted range the gap between the shroud and the filter elements is designed to provide a desired gas velocity. Further, it is advantageous to arrange the cluster of filter elements so that said cluster comprises a duct supported from a support- ing plate, the supporting duct can also function as a fluid communication path for gases, and a plurality of filter ele¬ ments are attached to said duct preferably in a manner which provides a space, eg. an annular space, between the shroud and the supporting duct being favourable for downward move- ment of the flushed solids. This is preferably accomplished by adapting the filter elements to the duct so that they are substantially enclosed by said duct so that the filter elements are attached to the duct in the vicinity of the area of said filter element receiving the gas to be purified. Thus the falling solids are allowed to slide smoothly along the surfaces of the shroud and the duct instead of accumulating on the surfaces of the filter elements. The shroud may be made of several materials, depending on the requirements of the environment, eg. when filtering gases of high temperature, say 1200°, a high temperature durable material such as metal for high temperatures or a ceramic material is required. It may be advantageous in some applications, eg. when the wall of the shroud is solid, to have the diameter of the tubular shroud changing in order to equalize the gas velocity in the space between the filter and the shroud, one alterna¬ tive is to form the shroud in conical shape, so that the diameter increases from the value at the midpoint towards the upper and lower ends of the shroud, this is the case when the gases are entered to the space between the shroud means and the filter through both ends, which, in fact, is a preferred solution due to eg. even distribution of the gases over the filter area. Alternatively, the shroud may be provided with openings, formed so that the downwardly flowing flushed material is not allowed to exit the shroud. Since the solids are easily entrained by the gases, it is preferred to have at least one of the shroud means to extend farther on its lower end than the others. That particular feature ensures that the upper level of a cloud of solids in a lower section of the first plenum of the filtering means is low enough not to be influenced by the gas flow to the filters during normal operation. So, it is ensured that the filters can not receive the solids already once filtered from the gases and removed from the filter. According to still another preferred embodiment of the present invention the filter elements are arranged to a substantially vertical wall preferably arranged side by side into a plurality of vertical rows. In this embodiment the backflushing may be arranged so as to be provided to each vertical row at a time, and thus the shroud means is adapted to each of said rows corresponding to the backflushing arrangement. The shroud means is preferably formed of a member, adapted to a proximity of the area of the filter element used for filtration, having an impingement portion for receiving the solids flushed from the filter element, and a material collecting portion for collecting the solids and conveying the solids to the lower section of the vessel. The width of said shroud means preferably substantially equals to or is greater than the width of the effective frontal area of the filter element in order to be able to collect the stream of solids flushed from the filter elements. The material col- lecting portion extends from the horizontal edges of the impingement portion, said collecting portion being preferably bent towards the filter element and provided with a bayment arrangement at the edge of said material collecting portion for preventing the solids from being entrained by the gas. The present invention is also applicable for a filter utilizing tubular filter elements instead of clusters of filter elements, since effective results are obtained in connection with tubular filters, too. In such an application each tubular filter element may be provided with a shroud means of its own, or two or more filters utilize the same shroud means; however, if too many filters are using the same shroud means, the effect will decrease. The tubular filter element may be a ceramic candle filter, or a conventional bag filter. DESCRIPTION OF THE DRAWINGS In the drawings: FIG. 1 is a sectional side view of the filtering ap¬ paratus using clusters of ceramic filter elements according to one preferred embodiment of the invention. FIG. 2 is a sectional plan view of the filtering ap¬ paratus using clusters of ceramic filter elements according to one preferred embodiment of the invention. FIG. 3 is a sectional plan view of the filtering ap¬ paratus according to another preferred embodiment of the invention. FIG. 4 is a sectional plan view of the filtering ap- paratus according to still another preferred embodiment of the invention. FIG. 5 is a sectional plan view of the filtering ap¬ paratus according to still another preferred embodiment of the invention. FIG. 6 is a view of the a cross section of the shroud means according to the preferred embodiment of the present invention. FIG. 7 is a view of the a cross section of the shroud means according to another preferred embodiment of the present invention. FIG. 8 is a top view of the filtering apparatus using clusters of ceramic filter elements adapted to a substan¬ tially vertical wall according to still another preferred embodiment of the invention. FIGS. 9 and 10 are perspective views of shrouds having conical or hourglass configurations. FIG. 11 is a side schematic view, with the near wall removed for clarity of illustration, of another embodiment of filtering apparatus according to the invention with a plurality of vertically spaced clusters of filter candles separated by shroud walls having a zig-zag configuration. FIG. 12 is a schematic cross-sectional view of the apparatus of FIG. 11 taken along lines 12-12 thereof. FIGS. 13 and 14 are detail views like that of FIG. 11 showing other arrangements of filter candles and shrouds according to still another aspects of the present invention. DETAILED DESCRIPTION OF THE PRESENT INVENTION Referring to FIG. 1, a filtering apparatus 1 according to a preferred embodiment of the present invention comprises a vessel 2 of preferably generally circular cross section having a support plate 3 dividing the vessel into two compartments: a first 6 and a second 7 plenum. An inlet 4 for gases to be purified and an outlet 13 for separated solids are adapted to be in connection to said first plenum and an outlet 5 is connected to be in connection to said second plenum 7 for discharging the gases. An assembly 11 for introducing a pulse of cleaning gas into each of clusters of filter elements is also provided, said clusters of filters 8 being supported by the supporting plate 3. The clusters are attached to the supporting plate preferably on their upper portion. Each cluster of filter elements 8 comprises a duct 12 preferably forming a flow path for gases from filter elements, attached to said duct, to the second plenum 7. The filter elements 9 are attached to the duct preferably on a gas receiving end of the filter element. By the expression "gas receiving end" is meant that end of the filter element in which front area receiving the gases containing contaminants is located. So, the filter element is actually attached substantially inside the duct. By that mea¬ sure, since the outside surface of said duct is created as smooth as possible, an easy sliding of solids flushed from the elements is thus ensured, and the advantageous perfor¬ mance is obtained. Since the surface of the filter element is considerably rough, the upper surface of each element especially can accumulate solids on them if the filter elements were not attached according to the present invention. As illustrated in FIG. 1 the supporting duct 12 is a tubular member, but if more gas space is required between the duct 12 and the shroud 10, the duct, being generally of smaller diameter, may be provided with local extension parts for every filter element, which are preferably to be fixed so that the elements are to be attached to the duct extension preferably on a gas re¬ ceiving end of the filter element, thus maintaining a smoother outer surface. At least one of the shroud means 10 can extend farther towards the lower section of the first plenum as indicated by reference 14 in order to ensure that the upper level of a cloud of solids in the lower section of the first plenum of the filtering means is low enough not be influenced by the gas flow to the filters during the normal operation. Thus the overall filtering operation is more efficient. As seen schematically at reference numeral 15 in Fig. 1, the inlet 4 may be connected to a circulating bed reactor, or a like source of high temperature, high pressure gas with entrained particles. Typically, the pressure of the gases discharged from the CFB reactor 15 and fed to the inlet 4 to vessel 2 is between about 2-40 bar, and the temperature is between 500-1200°C In this case, it is necessary for the filters 9 to be of ceramic or other high temperature resis¬ tant material, such as shown in U.S. patents 4,764,190, 4,793,292, 4,869,207, e.g. ceramic tube filters or monolithic ceramic filters in a generally tubular or parallepiped configuration. The gases to be filtered are introduced to the first plenum 6 via an inlet 4. Being conveyed to the space between the duct 12 and the shroud 10 preferably through both ends of the shrouds 10, and further to the filter elements the gases are fed through said elements to the interior space of the duct in order to separate the particulate from said gases. Thus, cleaned gases are further conveyed to the second plenum and discharged from there via outlet 5 for further proces- sing. When the assembly for supplying a flushing pulse of gas 11 is activated, a pulse of gas is introduced at least into one duct. Due to the reverse flow of gas through the elements the accumulated solids are dislodged from the filter. The dislodged solids impact the shroud, thus ensuring that the solids do not get influenced by the gas flowing to other filter clusters, and are conveyed to the lower section of the vessel to be removed via outlet 13. In FIG. 2 a filtering apparatus according to a preferred embodiment is shown as top view illustrating the assembly of the cluster 8 of filters in the vessel 2. The elements 9 are attached to the duct 12 from their gas receiving end, in this case totally inside the duct. The shroud 10 is adapted to form an annulus 21 between the duct and the shroud. The shroud may be formed also of non-circular elements, as shown in FIGS. 3 and 4. The shroud may be formed to a shape of polygon 410 thus enabling the usage of common walls 442 of two parallel shrouds. The duct 441 may be formed to a shape of a polygon, as well, but tubular form 412 is prefer¬ red. In some cases a form of a honeycomb or equivalent, in shroud construction 310 may be advantageous. A filter assembly 312 may be similar to the embodiment of FIG 1, but also a fabric bag filter may be applied as more clearly illustrated in FIG 5. The shroud 510 is formed of a plurality of adjacent square channels defining the space 521 between each filter 58 and the wall of the shroud. The filter 58 in this embodiment is preferably a ceramic candle tube, but may also be a fabric bag filter if qualities of a ceramic structure are not required for some reasons. Referring now to FIGS. 6 and 7, construction of a wall of the shroud 610 and 710 respectively is illustrated. It may comprise openings 661 and 771. In a case of the FIG 6 the wall is made of a refractory material such as ceramic ma¬ terial and openings 661 are provided to enhance the dis¬ tribution of the gas, however the openings are preferably directed so that the inner end of the opening is lower than the outer end. Thus the falling solids do not enter outside through the openings but slide towards the lower section of the vessel. The openings may be circular opening or slots, as well. In FIG. 7 another embodiment of the opening is illu¬ strated where the openings 771 are made by providing two bendings in sequence to the shroud wall. FIG. 8 discloses still another embodiment of the present invention. The filter elements 89 are attached to a substan¬ tially vertical wall 881 which may be cooled or non-cooled, in parallel, vertical rows, each row being provided with a backflushing assembly 882. The pattern of mounting the filter elements and the shroud means may, of course, vary according to the needs of a specific situation as long as the purpose of the shroud means is fulfilled. The shroud means 883, 884 and 885 are disposed adjacent the area of each filter discharging the solids during the cleaning mode operation. Shrouds 883 and 884 show two different solutions for filters having their gas receiving area at the end wall of said filter, but shrouds 885 may also be positioned at the sides of the filter element in case the gas receiving areas of the filter are at its side walls. The shroud is formed of an impingement portion 886 for receiving the solids flushed from the filter element, and a material collecting portion 887 for collecting the solids and conveying the solids to the lower section of the vessel. The material collecting portion extends from the horizontal edges of the impingement portion, said collecting portion is preferably bent towards the filter element and is provided with a bayment arrangement at the edge of said material collecting portion for preventing the solids to be entrained in the gas. FIGS. 9 and 10 show two other possible configurations of shrouds that may be utilized according to the invention (e.g. in place of shroud 10 in FIGS. 1 and 2). The shroud 70 in FIG. 9 comprises a generally conical (truncated cone) wall 71 with open top 72 and open bottom 73, normally gas passing through open top 72 into the filters (not shown) within shroud 70, and particles discharged through 73. The shroud 76 in Fig. 10 has an hourglass-simulating shaped wall 77, with an open top 78 and open bottom 79 into both of which gas to be filtered flows, with particles passing through bottom 79 when dislodged from filters (not shown) within shroud 76. The apparatus of FIGS. 11 and 12 illustrates an embodi¬ ment having a different configuration of shrouds and filters than those illustrated earlier. The filtering apparatus 101 includes a generally vertical vessel 102 within a dirty gas inlet adjacent the top at one side communicating with the interior volume (including portion/plenum 113) of the vessel 102 surrounding the conventional ceramic filter candles 109 disposed horizontally therein in vertically spaced clusters 108. The gas passes through the filter candles 109 into the clean gas ducts/headers/plenums 115 (see FIG. 12) associated with on or more clusters 108, passing out the clean gas outlet 105 common to all of the ducts 115. A reverse pulse of cleaning gas is added by means 111 (see FIG. 12), periodical¬ ly, intermittently, in response to a pressure build-up indicating clogging to dislodge filtered-out particles that have collected on the outer surfaces of the filters 109. In FIGS. 11 and 12, the shrouds 110 are illustrated in the form of walls (e.g. high temperature resistant metal, or ceramic, or metal covered with ceramic), except for the uppermost and lowermost shrouds 110, the walls of having a generally zig-zag configuration. The portion of each shroud 110 below a cluster 108 - e.g. walls 112 in FIG. 11, - is slanted with respect to the horizontal, downwardly flow toward the ash collector duct 114, to direct the flow of solids dislodged from the cluster 108 of the filters 109 immediately thereabove toward the bottom 106 of the vessel 102, to ultimately be discharged through the filtered-out particles outlet 113. FIGS. 13 and 14 show configurations like those in FIGS. 11 and 12, only having slightly different arrangements of the filters 109 and/or shrouds 110. In FIG. 13, the horizontally disposed filters 109 in each cluster 108 are disposed in an arrow tip configuration, and thus the shrouds 110 covering them also have such a configu- ration, the walls 112 serving to call dislodged filtered-out solids from the next-above cluster 108 to flow downwardly toward the particles outlet 113. In FIG. 14, the shrouds 110 are configured as illustrated in FIG. 11, however some of the filters have a different configuration. For the lower cluster 108 in FIG. 11, the ceramic filter candles 118 are disposed vertically, connected at the tops thereof to ducts 117, which in turn are connected to header 115, and ultimately to clean gas outlet 105. Using the apparatus described above, a method of filte- ring particles from gas (e.g. a high temperature and pressure gas from the pressurized CFB 15) is provided. The method comprises the steps of: a) Introducing gas with entrained particles to be filtered into the vessel 2, 102 into operati¬ ve association with the filters 9, 109, 118, etc. to filter particles from the gas and discharging cleaned gas from the vessel (through outlet 5, 105). b) At predetermined intervals (e.g. intermittently, periodically, in response to a back pressure indicating clogging, etc.), directing a pulse of cleaning gas toward at least some of the plurality of filters to dislodge filtered particles which have collected thereon, c) Preventing the particles dislodged from the filters 9, 109 etc. from being entrained in gas passing into association with other filters (in other clusters 8, 108). And, d), directing the dislodged particles so that they pass toward the particles discharge outlet 13, 113. Step c) is preferably practised by providing shrouds surrounding those portions of the filters 9, 58, 89 from which the particles will be dislodged, such as the shrouds 10,110, 310, 410, 510, 610, 710, 70, 76, or 883-887. Discharge of the particles from the vessel (2, through discharge 13) may be facilitated by mounting the clusters 8 etc. in ducts 12, 312, 412 etc. so that the outer surfaces of the ducts (surrounded by the shrouds 10 etc. ) are substantially smooth so that particles will readily flow down them toward the particles discharge (13). Thus it will be seen that according to the present invention a method and an apparatus are provided which provide enhanced filtering efficiency by substantially avoiding having to separate a particle from the gas to be filtered more than once. It is not intended to restrict the scope of the invention with the above detailed illustrations but all variations equivalent and obvious to a person skilled in the art are to be included within the scope of the protection described in the following claims.

Claims: What is claimed is: 1. An apparatus for filtering gases, essentially consist¬ ing of a vessel consisting of walls defining said vessel having at least a first plenum for gases containing contaminants to be removed from the gases and at least a second plenum for receiving gas from filtering means, at least one inlet for introducing gases into the first plenum, at least one outlet for filtered gases in the second plenum, at least one outlet for solids separated from gases in the first plenum, at least one filtering means adapted in the vessel form- ing a flow path for gases from the first plenum to the second plenum, which filtering means comprising at least one element substantially permeable to gases and substantially imperme¬ able to solids, through which element the gases are arranged to flow, means for supplying a reverse gas pulse into the fil¬ tering means for removing the accumulated solids separated from the gases flowing through the filtering means, and a shroud means surrounding at least part of the filtering means for preventing the re-entrainment of the solids removed from the filtering means. 2. An apparatus according to claim 1, wherein the shroud means is arranged so that the filtering element is totally surrounded from vertical portions thereof by the shroud means. 3. An apparatus according to claim 2, wherein the shroud means is formed so that the openings are arranged to upper and/or lower end of the shroud means. 4. An apparatus according to claim 2, wherein the shroud means is arranged to be attached to another shroud means from its outer surface. 5. An apparatus according to claim 2, wherein the shroud means is formed of a plurality of substantially parallel passage ways enclosing the filtering means, each passage way having at least one common wall with another passage way. 6. An apparatus according to claim 5, wherein the passage ways are arranged to be in a form of a honeycomb structure. 7. An apparatus according to claim 1, wherein the shroud means is a member, adapted to a proximity of area the filter element used for filtration, having a impingement portion for receiving the solids flushed from the filter element, and a material collecting portion for collecting the solids and conveying the solids to the lower section of the vessel. 8. An apparatus according to claim 7, wherein said shroud means comprises the impingement portion having a width substantially equal or greater to the width of effective front area of the filter element, and the material collecting portion extending from a horizontal edges of the impingement portion, said collection portion bent towards the filter element and having a bayment at the rearmost of said material collection portion for preventing the solids to be entrained in the gas. 9. An apparatus according to claim 1, wherein said shroud means comprises a wall provided with openings. 10. An apparatus according to claim 1, wherein said shroud means is of high temperature durable metal or a ceramic material. 11. An apparatus according to claim 1 and 12, wherein at least one shroud means is extending farther towards the lower section of the vessel compared to the other shroud means. 12. An apparatus for filtering gases, having a vessel consisting essentially of at least a first plenum for gases containing contaminants to be removed from the gases and at least a second plenum for receiving gas from filtering means, at least one inlet for introducing gases to the first plenum, at least one outlet for filtered gases, at least one outlet for solids separated from gases, means for supplying a reverse gas pulse into at least one of the filtering means in cleaning mode for removing the accumulated solids separated from the gases while the other filtering means are in their normal operating mode, at least one filtering means adapted in the vessel, which filtering means comprises at least one element substantially permeable to gases and substantially impermeable to solids, through which element the gases are arranged to flow, at least one duct forming a flow path for gases from the filter elements attached to said duct to the second plenum, said duct substantially enclosing the filter ele- ments, a shroud means for preventing the re-entrainment of the solids removed from the filtering means during a cleaning mode. 13. An apparatus according to claim 12, wherein the said duct encloses the filter elements. 14. An apparatus according to claim 13, wherein the said duct is provided with extensions enclosing the filter ele¬ ments. 15. An apparatus according to claim 2 and 12, wherein the shroud means is a tubular member having an inner diameter greater than one required to circumscribe the filtering means. 16. A method of operating a filtering device consisting essentially of: a vessel having at least a first plenum for gases con¬ taining contaminants to be removed from the gases and at least a second plenum for receiving gas from filtering means, at least one inlet for introducing gases into the first plenum, at least one outlet for filtered gases in the second plenum, at least one outlet for solids separated from gases in the first plenum, at least one filtering means adapted in the vessel form¬ ing a flow path for gases from the first plenum to the second plenum, which filtering means comprising at least one element substantially permeable to gases and substantially imperme- able to solids, through which element the gases are arranged to flow, means for supplying a reverse gas pulse into the fil¬ tering means for removing the accumulated solids separated from the gases flowing through the filtering means, said method comprising: introducing the gases containing contaminants to be removed from the gases via the inlet into a first plenum of said vessel; arranging said gases to flow through said filtering means, thus performing a filtering of said gases; separating and accumulating at least a portion of the contaminants from the gases into said filtering means; discharging the filtered gases via the outlet for further processing; arranging a flushing reverse gas pulse into at least one filtering means for removing the accumulated solids separated from the gases flowing through the filtering means; arranging at least portion of the removed solids to impact the shroud means; arranging said solids to be conveyed by said shroud means towards lower portion of said vessel.

Inventors: LEHTONEN PEKKA, MARTIN GARY, LEHTONEN, PEKKA, MARTIN, GARY
Applicants: AHLSTROEM OY, A. AHLSTROM CORPORATION
Application year: 1994
Main classification: B01D46
IPC codes: B01D46